The Catholicos of All Armenians is the chief bishop and spiritual leader of Armenia's national church, the Armenian Apostolic Church, and the worldwide Armenian diaspora. The Armenian Catholicos is also known as the Armenian Pontiff and by other titles. According to tradition, the apostles Saint Thaddeus and Saint Bartholomew brought Christianity to Armenia in the first century. Saint Gregory the Illuminator became the first Catholicos of All Armenians following the nation's adoption of Christianity as its official religion in 301 AD. The seat of the Catholicos, and the spiritual and administrative headquarters of the Armenian Church, is the Mother See of Holy Etchmiadzin, located in the city of Vagharshapat.

The Armenian Apostolic Church is the national church of Armenia. Part of Oriental Orthodoxy, it is one of the most ancient Christian institutions. The Kingdom of Armenia was the first state to adopt Christianity as its official religion under the rule of King Tiridates III, of the Arsacid dynasty in the early 4th century. According to tradition, the church originated in the missions of Apostles Bartholomew and Thaddeus of Edessa in the 1st century. St. Gregory the Illuminator was the first official primate of the church. It is sometimes referred to as the Armenian Apostolic Orthodox Church, Armenian Church or Armenian Gregorian Church.